A thermoluminescent dosimeter (TLD) is a type of radiation dosimeter. When a TLD is exposed to ionizing radiation at ambient temperatures, the radiation interacts with the phosphor crystal within the TLD, and deposits all or part of the incident energy in that crystal. Some of the atoms in the crystal that absorb that energy become ionized, producing free electrons, and these free electrons are trapped by imperfections in the crystal lattice structure. Heating the phosphor crystal causes the crystal lattice to vibrate, and releases the trapped electrons in the process. The released electrons return to the original ground state, and the captured energy from ionization is emitted as light, hence the name thermoluminescent. The emitted light is counted by photomultiplier tubes within a TLD card reader, and the number of photons counted is proportional to the quantity of radiation striking the phosphor crystal. The two most common types of TLD phosphor are calcium fluoride and lithium fluoride. The calcium fluoride TLD is used to record gamma exposure. The lithium fluoride TLD can be used to detect gamma exposure or neutron exposure indirectly [1].
When a thermoluminescent element is heated from some low temperature to some high temperature (e.g. above 400° C.), the intensity of the luminescence increases at first with more electrons in the traps are released, and then decreases when the number of trapped electrons decreases. This gives rise to a peak in the luminescence, which appears at a certain temperature. If there are several types of traps, several peaks are observed at different temperatures. This graph of the luminescence intensity as a function of temperature is called a “glow curve”. The heights of the peaks or the integrated area under the glow curve are found to depend on the radiation exposure dose. In a simple case, the dependence may be linear, which allows the radiation dose to be obtained from a measurement of the glow curve, after a proper calibration. This is the principle of thermoluminescence dosimetry (TLD) [1].
The electrons can also drop back to ground state after a long period of time. This effect is called fading, and is dependent on the incident radiation energy, and intrinsic properties of the TLD material. As a result, each crystal material possesses a limited shelf life after which dosimetry information can no longer be obtained. The shelf life of TLD material varies from several weeks in calcium fluoride to up to two years.
Temperature profile is a key process to evaluate TLD material for its dosimetry properties. Reliable results are obtained only when each TLD element is heated in a controlled and reproducible fashion (i.e. using the same initial and final temperatures and the same temperature variations during heating.) The most desirable way is to heat the elements linearly, so that for each element, the temperature follows the formula Te1=T0+αt, where T0 is the initial temperature, Te1 is the temperature of the sample at time t, and α is the heating rate. It is advantageous to heat the sample as fast as possible, since in this case the glow peaks are sharp and easy to measure, and more samples can be measured in a given time period. Thus, it is important to assure that heating of the TLD elements accurately reflects the expected heating rates.
Several methods have been used in the past for heating the crystal (TLD element), which can be separated into three large categories, including contact heating, optical heating, and hot gas heating. Exemplary methods and systems may be found in a number of prior art publications, for example in U.S. Pat. Nos. 3,531,641, 3,729,630, 3,975,637, 4,204,119, 4,638,163, 4,835,388, 4,839,518, 5,041,734, 5,081,363, 5,606,163, 6,005,231 and 6,414,324.
Traditional contact heating uses tiny heaters placed in close contact with the crystal (TLD element), with the heater temperature controlled. Contact heating is highly non-uniform and relatively slow. The results are often non-reproducible, because the temperature of the TLD element depends on the thermal contact between the crystal element and the heater, repeat heating also limits a dosimeter's useful life. The contact pressure must be periodically checked, and adjusted to ensure proper thermal contact in order to eliminate incomplete readouts. In an optical heating method, the crystal element is heated via absorption of radiation emitted from an incandescent or laser source by either the crystal element or the substrate on which the crystal element is placed. Readout cycle using optical heating is fast, but the reproducibility of optical heating method is poor, especially with substrate heating. In addition, optical heating method requires the TLD chip (i.e. chip containing the TLD elements) to be thin and small, which results in reduced sensitivity for the dosimeter. In gas heating method, the crystal (TLD element) is heated by a stream of hot gas with controlled temperature. This method provides more uniform and faster heating, but requires a complex gas heating system, which is more expensive.
In all three heating methods, the temperature of the heating source is controlled but the real time temperature of the TLD element or the environmental chamber temperature is not measured to ensure accurate TLD heating. Currently, the verification of heating of a TLD element is done periodically or upon request, by the manufacturer during the TLD calibration process. The calibration process requires the TLD card reader to be opened up, and the temperature data directly obtained from the heater electronic board signal |ports|[YNCN1]. Because the temperatures measurements of the heater are taken while the device cover is open, the readings of these temperature measurements do not accurately reflect the true temperature profile of the TLD in operation. It also requires considerable downtime of the machine.
U.S. Pat. No. 7,439,524, to Abraham Katzir, describes an optical heating thermoluminescence-based dosimetry system, which is equipped with an infrared radiation (IR) radiometry subsystem for real-time control of heating of the TLD element. The IR radiometry subsystem monitors IR radiation emitted from each TLD element surface during the heating cycle. The respective IR radiation inputs are then converted to temperature of each TLD element, which are then used to control the laser that is heating the TLD element. While the Katzir device is capable of measuring the real-time temperature of each TLD elements during the heating cycle, the IR radiometry subsystem requires complex and delicately arranged optical pathway, using mirrors and lens or special optical fibers to collect and focus the IR radiation emitted from the surface of each TLD element to respective IR detector. The infrared detectors used for this IR radiometry subsystem also require a clean operating environment without dust and high humidity, which may limits its application in machines that utilize other heating methods. Accurate IR detectors are expensive. The hardware requirements of the IR radiometry subsystem may thus significantly increase the size and cost of the TLD card reader, strict its placement, transport and operation environment, and affects its reliability.
In conclusion, all prior art methods are disadvantageous in that the temperature of the TLD elements is not well monitored consequently, glow curves suffer from irreproducibility and so do dosimetry results. Therefore, it would be highly advantageous to have a TLD temperature monitoring system and method in which the temperature and heating rate of each TLD element is easily measured, and calibrated in real-time, which is also cost-effective, operational and reliable.
The primary advantage of the inventive device and method is that it allows for daily verification of the reader's temperature profile without any machine downtime or significant machine modification or additional hardware. It also allows inter comparison between readers to ensure that the same TLD card is appropriately read on separate readers.